Related fields include semiconductor devices and their fabrication; in particular, thin-film components of resistive-switching non-volatile memory (ReRAM).
Nonvolatile memory elements are used in computers and other devices requiring persistent data storage (e.g., cameras, music players). Some traditional nonvolatile memory technologies (e.g., EEPROM, NAND flash) have proven difficult to scale down to smaller or higher-density configurations. Therefore, a need has developed for alternative nonvolatile memory technologies that can be scaled down successfully in terms of performance, reliability, and cost.
In resistive-switching-based nonvolatile memory, each individual cell includes a bistable variable resistor. It can be put into one of at least two states (low-resistance or high-resistance), and will stay in that state until receiving the type of input that changes it to the other state (a “write signal”). The resistive state of the variable resistor corresponds to a bit value; e.g., the low-resistance state may represent logic “1” and the high-resistance state may represent logic “0”. The cell is written by applying a write signal that causes the variable resistor to change resistance. The cell is read by measuring its resistance in a way that does not change it.
Preferably, write and read operations should require as little power as possible, both to conserve energy and to avoid generating waste heat. Lowering the cells' entire range of resistance states, while keeping the different states distinguishable, can lower the required operating power.
Many ReRAM devices change resistance by creating and destroying, or lengthening and shortening, one or more conductive paths through a variable-resistance layer or stack while the bulk material remains static (e.g., it does not change phase). The bulk material is often a highly insulating dielectric. The conductive paths (also known as “percolation paths”) are formed when an electric field organizes conductive or charged defects or impurities into a filament stretching from one interface to the other, with sufficient defect density that charge carriers can easily traverse the layer by tunneling from defect to defect. To return the variable resistor to the high-resistive state, it is often not necessary to destroy the entire filament, but only to introduce a gap too wide for tunneling somewhere along the filament's length. Some of the types of defects that have been used include metal clusters and oxygen (or nitrogen) vacancies.
The “forming operation” that creates the very first filament in a newly fabricated ReRAM cell is risky. The risk is of “over-forming;” creating a filament so wide or dense that the operating write signals are too weak to break it. An over-formed cell cannot be rewritten. At this point the entire device has been built, so the investment has been significant and the cost of failure is high. To prevent over-forming, resistive layers are added to the cell, functioning as resistors connected in series to limit current in the variable-resistance stack during the forming operation. The higher the added resistance during forming, the more the variable-resistance stack is protected. However, as discussed above, the higher the added resistance during normal writing, the more power is required to write the cell.
Therefore, a need exists for a practical, cost-effective way to protect the variable-resistance stack from over-forming during the forming operation, yet enable the cell to operate at low power for subsequent reading and writing.